The present invention concerns the manufacture of ethylene oxide. More precisely, the invention concerns a process and apparatus system for concentrating aqueous ethylene oxide solutions, such as, for instance, those solutions obtained by water absorption of ethylene oxide in order to separate it from the reaction mixture of synthesis.
On an industrial scale, ethylene oxide generally is prepared by the oxidation in the gaseous phase of ethylene by oxygen in the presence of a catalyst based on silver. Since the ethylene oxide is very dilute in the gaseous mixture produced from the oxidation reaction, this mixture must subsequently be subjected to various treatments (absorptions, distillations, flash, and the like) in order to obtain pure ethylene oxide (see for instance French Pat. No. 1,343,492, French Pat. No. 2,305,436 and U.S. Pat. No. 3,904,656).
In practice, the isolation of ethylene oxide from the gaseous mixture obtained from the synthesis is carried out in several steps:
(a) Water Absorption: The gaseous mixture is placed in contact with water in a column including several theoretical stages or gas-liquid contact devices. In this matter, an aqueous solution is obtained containing about 2.5% by weight of ethylene oxide, as well as dissolved gases (CO.sub.2, CH.sub.4, C.sub.2 H.sub.4, nitrogen, argon, etc.) and other impurities (principally formaldehyde and acetaldehyde);
(b) Desorption: The above aqueous solution is "stripped" in a distillation column, with or without an enrichment section. At the base one obtains an aqueous stream no longer containing ethylene oxide and at the top a mixture of ethylene oxide, steam and dissolved gases and other impurities initially present in the aqueous ethylene oxide solution. This gaseous stream has an ethylene oxide content of about 30 to 60% by weight;
(c) Reabsorption: The preceding gaseous stream is cooled, then placed in contact with water in order to reabsorb the ethylene oxide. The greatest part of the dissolved gases is not reabsorbed in the water and is easily separated in the form of a gas stream. An aqueous ethylene oxide solution is obtained whose concentration is between 5 and 15% by weight; and
(d) Distillation: The solution is then distilled in order to obtain pure ethylene oxide.
The energy required in order to separate the two constituents from this solution increases as the ethylene oxide content decreases. Thus, while the gaseous mixture issuing from step (b) contains about 30 to 60% by weight of ethylene oxide, water must be readded in order to reabsorb the ethylene oxide and this step subsequently requires a considerable amount of energy in order to separate this water from the ethylene oxide.